The forward converter is the preferred power supply topology for many applications, including providing power to telecommunications equipment. Forward converters provide a regulated output voltage to a load which is smaller than the input voltage supplied by an associated input power supply. A conventional forward converter 10 is illustrated in FIG. 1. As shown in FIG. 1, a power switch S1 is coupled in series with the primary winding 14 of a transformer. Coupled in parallel across the primary winding 14 is an RCD network comprising a resistor 16, a capacitor 17 and a diode 18. The RCD network is used to reset the transformer core of the converter by recycling the magnetizing energy from the primary winding 14 of the transformer back to the input supply of the converter during the off period of S1. It also limits the peak voltage across S1. The secondary side of the converter 10 includes a forward rectifier 20 coupled to the secondary winding 15 of the transformer, a free-wheeling rectifier 22 and an output filter consisting of choke inductance 24 and capacitor 26. The output filter provides a substantially dc output voltage to a load R.sub.L.
The conductivity of S1 is controlled by applying a suitable waveform to the gate 12 of S1. The waveform applied to the gate 12 of S1 is typically provided by a feedback control circuit (not shown) which supplies a pulsed control signal using pulse width modulation (PWM), for example, to regulate the output voltage level. When S1 is turned on, i.e. conducting, the input voltage, V.sub.IN, is applied across the primary winding 14 of the transformer. A secondary voltage V.sub.S is developed across a secondary winding 15 of the transformer and is applied across the forward output rectifier 20. Current and power flows to the choke inductor 24 and output capacitor 26 (which forms an LC output filter) and the load, R.sub.L. Assuming the output capacitor 26 is sufficiently large and ignoring diode drops and losses, the voltage across the choke inductor 24 will be equal to V.sub.S -V.sub.OUT. In this fashion, the current flowing in the choke inductor 24 will increase linearly with time and can be described by di.sub.L /dt=(V.sub.S- V.sub.OUT)/L.sub.O, where L.sub.O is the size of the choke inductor 24.
When S1 is turned off. i.e., rendered non-conducting, the secondary voltage V.sub.S will reverse. However, the current in the choke inductor 24 will continue to flow in the forward direction rendering the free-wheeling diode 22 conductive. This permits the current to continue to circulate in the circuit loop bounded by the free-wheeling diode 22, choke inductor 24, output capacitor 26 and load R.sub.L. The current in the choke inductor 24 then decreases with time and may be represented by di.sub.L /dt=-V.sub.OUT /L.sub.O.
A drawback associated with conventional forward converters, as described above, is that the RCD network connected across S1 can be a significant source of power loss. Power is lost in resistor 16 when switch S1 is on during the time that capacitor 17 is discharging, and when switch S1 is off during the time that a magnetizing energy is being returned to the input supply of the converter. The highest power loss is typically due to the discharging of capacitor 17 and this power loss increases with larger values of capacitor 17 capacitance. The value of capacitor 17 is selected to minimize the peak voltage as switch S1 turns off and to provide a half cycle reset via the transformer primary winding 14. Generally, the value of capacitor 17 is selected for the transformer reset function since it is greater than the value of capacitance required to minimize the peak voltage due to the effects of the leakage inductance of the transformer.